Sensors that rely on mechanical resonance such as quartz crystal microbalance (QCM) sensors, microcantilever sensors, and plasma resonance sensors (SPR) are commonly used in flow cells. A flow cell is a vessel in which a sensor is placed to detect material in a medium. Typically, the medium is pumped through the flow cell. Thus, the medium flows through the cell, and hence the moniker flow cell. Typically flow rates of the medium in flow cells utilizing mechanical resonance sensors are on the order of micro-liters per minute (μL/min). For example, the recommended liquid flow rate for use in instrument grade quartz crystal microbalance QCM sensors is 200 μL/min. Lower flow rates of about 33 μL/min are typically used with other microcantilever sensors.
In general, performance of both QCM and microcantilever sensors deteriorates at high flow rates. This can present a problem when the sample volume of the medium is large and the amount of target material (material to be detected) in the medium is low. For example, the tolerable level for Cryptosporidium and Giardia is 0.2 cells per liter of drinking water. Thus, using a typical QCM and microcantilever sensor to test a source of drinking water for parasites, such as Cryptosporidium (a parasite known to cause diarrhea) or Giardia (a parasite known to infect the intestinal tract), is neither effective or practicable without a concentrating step.